Self-consistent optical constants of LiF films

Lithium fluoride (LiF) has the shortest known cut-off in nature, with transparency down to 102.5 nm, which makes it a critical material for far-ultraviolet (FUV, 100 nm < λ < 200 nm) optical elements. Accurate optical constants are essential for element design. As with other metal fluorides, LiF film optical constants are temperature sensitive. This study presents optical constants for LiF thin films deposited by boat evaporation onto substrates at 298 K and 503 K. Reflectance and transmittance in the range of 30-195 nm were measured in situ, in order to avoid the instability of LiF films, particularly in contact with humid air. Ellipsometry measurements were carried out from 210 to 1690 nm and were fitted with a Lorentz oscillator. The self-consistent optical constants were obtained using the Kramers-Kronig (KK) analysis by extending the data range with literature data and extrapolations. Calculated results using the obtained optical constants show good agreement with the experimental measurements. Furthermore, the global self-consistency was assessed by a classical sum-rule at the whole range, and the local self-consistency by what we believe to be a novel sum-rule method incorporating window functions at each photon energy range, yielding satisfactory results. These are the first self-consistent optical constants reported for LiF films.